In this country there is a recurring need to clean the water mains or pipes which deliver potable water to residences or businesses. Consumers of potable water often notice the need to clean water mains by the observance of discolored water or reduced water pressure. When such discolored water or reduced pressure is noticed, the cleaning of water mains in the United States has been accomplished primarily by either flushing the water main with large quantities of clean water or physically passing a "pig" or a "swab" through the water main. In extreme cases, old water mains are dug up and replaced by new water mains.
Generally, the flushing of water mains has proven to be ineffective because undisturbed mineral scale or sediment in previously uncleaned water mains is loosened by the flushing water so that it may continue to enter the flow of water after the flushing has been completed. This entry of mineral scale or sediment into water mains oftentimes exacerbates the problems faced by water users. Comprehensive flushing programs also do little to resolve and prevent future water discoloration. In addition to wasting a tremendous amount of water, flushing only temporarily clears the center of the water main. This is due to the nature of water flow through a circular conductor such as a pipe. Because water flows most freely in the center of the pipe, very little flow occurs near the pipe walls. Therefore, flushing has little or no effect on mineral scale or sediment which builds up on the pipe walls. As mentioned earlier, flushing causes some mineral scale or sediment to actually re-enter the water stream and possibly produce additional discoloration or reduced pressure of the water provided to users.
The use of a mechanical "pig" or a "swab" has become the most commonly used method of cleaning water mains in the United States. Swabbing involves inserting a bullet-shaped "sponge" into the water main and then forcing the "sponge" one or more times through the water main using system water pressure. The insertion and extraction points for the sponge are typically fire hydrants. These fire hydrants must be disassembled to properly insert the sponge into the water main. The swabbing or pigging process is repeated on an isolated section of water main until all mineral scale and sediment is pushed out. The major benefit to swabbing or pigging is the high degree of cleaning achieved. There are, however, disadvantages to swabbing or pigging. Specifically, the swabbing or pigging process is costly and timely. This cost and time is related in part to the disassembly and reassembly of the fire hydrants to introduce the swab or the pig into the section of the water main to be cleared. Swabbing or pigging is also limited in its application as it cannot be performed on water mains which include "butterfly" or 90 degree pivot valves.
Professionals in the water distribution industry realize that periodic cleaning of water mains is a necessary part of the proper maintenance of a water distribution system. Therefore, in addition to responding to the complaints of consumers it is also necessary to make the cleaning of water mains a regularly scheduled maintenance event.
In the U.K., the cleaning of water mains has been accomplished by a process termed "air scouring." The process of air scouring involves isolating a section of a water main, and then purging the standing water from the isolated section of the water main with high volume, high velocity, low pressure compressed air. Slugs of water are produced through the passage of controlled amounts of water into the isolated section of the water main. The compressed air causes these slugs of water to swirl through the water main removing mineral scale and sediment. The water main is restored to full service following the process of air scouring.
While the basic system of air scouring has shown great promise for utilization in the United States, there are certain modifications that must be made to assure that the system is acceptable for use in the water distribution systems found in most U.S. cities, towns, municipalities, and rural areas.